Bug-fix: Mirror locks UI on exception

Added a try-finally block in mirrror_navi to make sure UI updates resume
even if an exception is thrown.
Selection management on MdiSubWindow activation

Updates data view TreeWidget, and calls action-specific callbacks if the
active MdiSubWindow changes. This should make one-click tools act more
intuitively (on active window).
Made PCA action only active for navigation dim > 0

Fixed format of selection callback arguments to include action, as
originally intended. Also implemented util function win2sig to get a
signal from window. (matches window to signal lists' signal_plots and
navigator_plots).